This invention pertains to a catheter designed to couple radiofrequency (RF) energy to biological tissue surrounding the catheter tip. Typical application is in thermal ablation of cardiac tissue.
Percutaneous ablation is a therapeutic procedure used with increasing frequency for treatment of ventricular tachycardia. It works by destroying cardiac tissue responsible for the disease. For example, this subject is covered in Ablation in Cardiac Arrhythmias, G. Fontaine & M. M. Scheinman (Eds.), Futura Publishing Company, New York, 1987. A recent review of this field is given in a chapter by D. Newman, G. T. Evans, Jr., and M. M. Scheinman entitled "Catheter Ablation of Cardiac Arrhythmias" in the 1989 issue of Current Problems in Cardiology, Year Book Medical Publishers.
Currently, catheter ablation is performed by delivering a high voltage direct current pulse from a standard defibrillator through an electrode catheter designed for temporary pacing. Radiofrequency (RF) ablation using electrosurgical power units is in clinical investigation, as a safer ablation alternative to high voltage direct current pulses. Continuous, unmodulated RF output in the frequency range of 500 KHz to 1.2 MHz is typically used. (RF without qualifiers refers to the electromagnetic spectrum from 10 kHz to 100 GHz.) Laser energy is also being tested for catheter ablation of arrhythmias.
Some experimentation has been reported with the use of microwave energy for catheter ablation. U.S. Pat. No. 4,641,649 issued Feb. 10, 1987 to P. Walinski, A. Rosen and A. Greenspon describes a catheter consisting of a miniature coaxial line terminated in a protruding inner conductor antenna. This system operates at 925 MHz. Another microwave ablation catheter experiment has been reported by K. J. Beckman, & J. C. Lin et al, "Production of Reversible Atrio-Ventricular Block by Microwave Energy" abstracted in Circulation 76 (IV): IV-405, 1987. Technical details of a folded dipole applicator catheter used by Beckman have been described by J. C. Lin and Yu-jin Wang in "An Implantable Microwave Antenna for Interstitial Hyperthermia" in Proceedings of the IEEE, Vol. 75 (8), p. 1132, August, 1987. An earlier microwave applicator which fits into a blunt-ended mylar catheter has been described by B. E. Lyons, R. H. Britt, and J. W. Strohbehn in "Localized Hyperthermia in the Treatment of Malignant Brain Tumors Using an Interstitial Microwave Antenna Array", IEEE Trans on Biomedical Engineering, Vol. BME-31 (1), pp. 53-62, January, 1984.
A general geometrical requirement of catheter-based applicators is that they must be confined in slender cylindrical structure with a radius commensurate with the catheter diameter. In the discussion of catheter applicators which follows, it is convenient to adopt a cylindrical coordinate system with the z-axis coincident with the catheter axis and pointed toward the distal end. The radial component is at the direction normal to the z-axis and the circumferential component has a direction around the z-axis. Radius "r" is measured from the catheter axis. The catheter diameter is "a".
A common feature of all of the above RF catheters (the laser catheter which is an optical device will not be discussed further) is that the energy delivery is predominantly via an electric field (E-field) originating at the applicator's electrode/tissue interface. This class of catheter applicator will therefore be referred to as E-field applicators. Although the configurations of the E-field applicators described above vary, the E-field coupling causes a rapid decrease in current density and therefore tissue heating, as a function of distance from the electrode.
In order to represent the state of the art of RF heating catheters and to compare it with the preferred embodiment of this invention, a simplified E-field applicator is shown in FIG. 1A. Applicator electrode 10 is a wire immersed in a lossy dielectric medium which has electrical properties typical of muscle tissue. In spite of the simple geometry and low frequency approximation used in the description, FIG. 1 retains the salient feature of an E-field coupling.
In FIG. 1A, RF potential V14 is applied between applicator electrode 10 and a remote boundary 15 which corresponds to a neutral electrode applied to the skin. The exact location of boundary 15 is not important to the shape of the E-field near applicator electrode 10. Radial electric field E16 coincides with current density vector J.sub.r =.sigma.E.sub.r in the tissue, where .sigma. is the conductivity of the tissue.
Continuity of current in a cylindrical geometry in FIG. 1 results in current density which decreases with the inverse square of the radius r. Therefore, corresponding electrical power dissipation resulting in heating of tissue decreases with the fourth power of a/r. Typically, an electrode radius is limited by practical catheter size to a maximum of 1 mm. In order to effectively ablate ventricular tachycardia (see Moran, J. M., Kehoe, R. F., Loeb, J. M., Lictenthal, P. R., Sanders, J. H. & Michaelis, L. L. "Extended endocardial Resection for the Treatment of Ventricular Tachycardia and Ventricular Fibrillation", Ann Thorac Surg 1982, 34: 538-43), it is desirable to heat tissue up to 10 mm from the catheter axis. In the applicator represented by electrode 10 in FIG. 1A, heat dissipation at the catheter surface is 10,000 times more intense than heat dissipation at a 10 mm radius.
In order to examine the effect of this wide range of heat dissipation, it is useful to divide the lossy medium in FIG. 1A into three cylindrical shells: first shell R11 adjacent to the applicator electrode 10, followed by shell R12, and R13 beginning at the 10 mm radius. Since the shells are traversed by the same current and the potential drop across the shells is additive, energy delivery can be represented by three resistances R11, R12, and R13 in FIG. 1B, connected in series with the source of RF potential V14.
A very steep heating gradient at the applicator electrode 10 tends to desiccate blood or tissue close to the electrode, increasing the resistivity of R11. This in turn further increases the relative power dissipation in R11 in comparison with R12 and R13, until effective impedance of the desiccated region R11 becomes, in effect, an open circuit shutting off the flow of RF power to the tissue beyond R11. This indeed is the problem of state-of-the-art RF ablation catheters which severely limits effective heat delivery to more distant tissue.
Insulation of the applicator electrode 10 from the tissue does not reduce the heat dissipation gradient: If the applicator electrode 10 is insulated from the lossy medium by a thin dielectric tube, the effect of the dielectric can be represented by capacitor (not shown) in series with the source of RF potential V14. Now the applicator must be operated at a frequency high enough so that the impedance of the sum of resistances R11 and R12 and R13 must be higher than the capacitive impedance of the dielectric tube. R11 still dominates the heat distribution.
Therefore in biomedical applications, there is a need for a catheter-compatible RF energy delivery system which dissipates heat more uniformly to a specified depth, thereby leading to a more accurately controlled and larger ablated region. It is also desirable to eliminate the effect of desiccation of tissue adjacent to the electrode on heat dissipation to surrounding tissue.